To a stirred solution of maleic thioanhydride 2a (2.1 g, 18 mmol) at 0 ^oC in dichloromethane (20 ml) was dropwise added a solution of trimethylphosphite (1.1 g, 9.0 mmol) in dichloromethane (10 ml) under nitrogen. After 0.5 h, the reaction mixture was evaporated under high vacuum, dissolved in toluene and rapidly filtered through silica with suction. Evaporation of the toluene gave a yellow oil which was dissolved in chloroform. Addition of hexane precipitated 0.37 g (9.7%) of a white powder melting at 103-104 ^oC.

¹ H NMR (200 MHz, [²H₆]acetone) (delta, intensity, multiplicity, J_HH, assignment): 4.16 ppm, 2, s, -, H3,4; 6.74 ppm, 1, d, 5.8 Hz, H8; 8.08 ppm, 1, d, 5.7 Hz, H7.

¹³C NMR (50 MHz, [²H₆]acetone) (delta, multiplicity, J_CH, assignment): 47.24 ppm, d, 196 Hz, C3,4; 51.58 ppm, s, -, C6; 137.11 ppm, d, 181 Hz, C8; 151.51 ppm, d, 177 Hz, C7; 193.15 ppm, s, -, C2,5; 195.36 ppm, s, -, C9.

MS (IP 70 eV, m/z > 96, intensity > 5%, assignment): 212, 5.1, M⁺; 153, 9.5, -; 152, 100, M⁺-COS; 126, 11, M⁺-COS-C₂H₂; 124, 24, M⁺-COS-CO; 98, 18, M⁺-COS-C₂H₂-CO; 96, 39, M⁺-COS-2CO.

FTIR (KBr): 3092 cm^-1, s, CH; 1848 cm^-1, s, CO (C2); 1782 cm^-1, s, CO(C5); 1742 cm^-1, s, CO(C9); 1683 cm^-1, s, CO (C9).

To a stirred solution of dichloro maleic thioanhydride (2b) (0.55 g, 3.0 mmol) in dichloromethane (4 ml) cooled in ice was dropwise added a solution of triethyl phosphite (0.50 g, 3.0 mmol) in dichloromethane (1 ml) under nitrogen. After 0.5 h a solid product was collected by filtration and washed with diethyl ether. This crude product (0.25 g, 52%) was recrystallized from p-xylene and then sublimed in vacuum to give 95 mg (20%) of light yellow crystals melting at 273 ^oC.

¹³C NMR (50 MHz, CDCl₃) (delta, multiplicity, assignment): 118.06 ppm, s, C10; 124.16 ppm, s, C9; 133.38 ppm, s, C3; 139.59 ppm, s, C4; 140.58 ppm, s, C7; 140.87 ppm, s C8; 153.85 ppm, s, C2; 173.63 ppm, s, C6.

MS (IP 70 eV, m/z > 199, intensity > 5%, assignment): 318 (4Cl), 92, M⁺; 290 (4Cl), 100, M⁺- CO; 283 (3Cl), 45, M⁺- Cl; 262 (4Cl), 64, M⁺- 2CO; 255 (3Cl), 11, M⁺- CO - Cl; 234 (4Cl), 39, M⁺- 3CO; 227 (3Cl), 12, M⁺- 2CO - Cl; 199 (3Cl), 33, M⁺- 3CO - Cl.

IR (KBr): 1736 cm^-1, s, CO (C2) ; 1618 cm^-1, s, CO (C6).

To a stirred solution of dichloro maleic anhydride (2c) (0.60 g, 3.3 mmol) in tetrahydrofuran (30 ml) at room temp. was dropwise added a solution of triphenylphosphine (0.86 g, 3.3 mmol) in tetrahydrofuran (10 ml) under nitrogen. After 0.5 h, a solution of water (60 mg, 3.3 mmol) in tetrahydrofuran (10 ml) was added to the reaction mixture. Evolution of hydrogen chloride was observed. After an additional 1 h, nitrogen was bubbled through the solution to remove all the HCl and the solvent was evaporated. The residue was recrystallized from toluene to give 0.91 g (71%) of 6b melting at 190-193 ^oC.

¹ H NMR (200 MHz, CDCl₃) (delta, multiplicity, assignment): 7.5 - 7.8 ppm, m, C_phenyl

¹³C NMR (50 MHz, CDCl₃) (delta, multiplicity, J_CP, assignment): 79.22 ppm, d, 110 Hz, C3; 121.15 ppm, d, 100 Hz, C5; 129.78 ppm, d, 14 Hz, C8, 10; 134.03 ppm, s, C9; 134.28 ppm, d, 12 Hz, C7,11; 181.74 ppm, d, 13 Hz, C4; 183.75 ppm, d, 19 Hz, C2; 191.09 ppm, d, 7 Hz, C5.

IR (KBr): 1740 cm^-1, C(5)O; 1690 cm^-1, C(2)O; 1650 cm^-1 C(4)O.